The present invention relates to a magnetic recording system, and more particularly a magnetic recording system having a recording density of 2 gigabits or higher and a thin film magnetic recording medium with low noises for realizing such system.
Demands for large capacity of a magnetic recording system are increasing more and more nowadays. Conventionally, an inductive head has been used which utilizes a voltage change caused by a change of a magnetic flux with time. This inductive head performs both read and write. Recently, use of a composite head has expanded rapidly which uses different heads for read and write by introducing a magnetoresistive head having a higher sensitivity as a read head. The magnetoresistive head utilizes the phenomenon that the electrical resistance of the head element changes with a leakage flux change of a medium. A head having a much higher sensitivity constituted of a plurality of magnetic layers laminated between a plurality of non-magnetic layers laminated between those magnetic layers is now under development which utilizes a very large magenetoresistance change (giant magnetoresistive effect or spin valve effect). This giant magnetoresistive effect is an electrical resistance change to be caused by a change in relative directions of magnetization of a plurality of magnetic layers interposed between non-magnetic layers. And the relative direction change is caused by leakage fluxes of a recording medium.
Magnetic layers of magnetic recording media presently used in practice are made of alloy whose main components are Co such as CoCrPt, CoCrTa and CoNiCr. Such Co alloy has a hexagonal closed packed structure (hcp structure) having a c-axis as a magnetic easy axis. It is therefore preferable that an in-plane magnetic recording medium has the crystallographic orientation having the c-axis oriented along the in-plane direction. However, such an orientation is unstable so that it cannot be formed if Co alloy is directly deposited on a substrate. The (100) plane of Cr having a body centered cubic structure (bcc structure) has a good lattice matching with the (11.0) plane of Co alloy. By using this good lattice matching, first an underlayer of Cr having the (100) plane is fabricated on a substrate, and a Co alloy layer is epitaxially grown on the Cr underlayer to thereby form the (11.0) plane having the c-axis oriented in the in-plane direction. In order to further improve the crystal lattice matching at the interface between the Co alloy magnetic layer and the Cr underlayer, a second element is added to Cr to increase an interstitial distance. The (11.0) crystallographic orientation of Co alloy therefore increases further and coercivity can be increased. Examples of these techniques are to add V, Ti, or the like as disclosed in JP-A-62-257618 and JP-A-63-197018.
Factors necessary for high density magnetic recording include low noises as well as high coercivity of recording media. Media noises are mainly caused by an irregular zig-zag pattern formed in magnetization transition regions between recording bits. It is necessary to smooth these transition regions in order to reduce media noises. It is known that fine magnetic crystal grains and uniform crystal grain sizes are effective for reducing media noises. To this end, it is effective to make fine and uniform crystal grains of the underlayer. The above-described known techniques increase the lattice constant of the underlayer by adding a second element to the Cr underlayer, but do not make fine and uniform crystal grains of the underlayer. Therefore, although the above techniques are effective for increasing coercivity, they are not effective for reducing media noises.
Significant requisites for magnetic disk media are improvement of shock resistance. This shock resistance improvement is a very significant issue from the viewpoint of reliability of magnetic disk media, particularly for magnetic disk drives mounted on recent portable information apparatuses such as note type personal computers. Instead of using an Al alloy substrate with an NiP plated surface (hereinafter called an Al alloy substrate), using a glass substrate with a reinforced surface or a crystallized glass substrate can improve the shock resistance of magnetic disk media. As compared to conventional Al alloy substrates, the glass substrate has a smoother surface so that it is suitable for high density recording because it is effective for reducing a flying space between the magnetic heads and the medium. The glass substrate is, however, associated with some problems such as insufficient adhesion relative to the substrate and permeation of impurity ions on the substrate or adsorbed gas on the substrate surface into the Cr alloy underlayer. Of these problems, the film adhesion property in particular is degraded if the glass substrate is heated as reported in J. Vac. Sci. Technol. A4(3), 1986, at pp. 532 to 535.
Countermeasures for these problems include fabricating a film such as a metal film, an alloy film and an oxide film between the glass substrate and Cr alloy underlayer (JP-A-62-293512, JP-A-2-29923, JP-5-135343).
As compared to an Al alloy substrate, a glass substrate of an in-plane magnetic recording medium has worse electromagnetic conversion characteristics at high linear recording density regions. The reason for this is as in the following. A Cr alloy underlayer fabricated on a glass substrate directly or via a film made of one of metals or its alloy described in the above conventional techniques, does not display strong (100) orientation compared to it is fabricated on an Al alloy substrate. Therefore, the crystal plane other than the (11.0) plane of the Co alloy magnetic layer grows parallel to the substrate surface and the in-plane orientation of the c-axis as, which is magnetic easy axis becomes small. From this reason, coercivity lowers and a read output at high linear recording density lowers. Furthermore, if the glass substrate is used, crystal grains in the magnetic layer become bulky than using an Al alloy substrate, and the crystal grain size dispersion becomes larger by about 20 to 30%. These are main reasons to increased media noises and degraded electromagnetic conversion characteristics of media using the glass substrate. JP-A-4-153910 discloses that the size of crystal grains of a magnetic layer can be suppressed from becoming bulky and the magnetic characteristics can be improved if an amorphous film being made of Y and one of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and the like is inserted between the glass substrate and underlayer.
With this method, however, although the size of crystal grains of the magnetic layer may be reduced to some degree, the in-plane components of the magnetic easy axis reduce and this is not sufficient for a magnetoresistive head to realize a high recording density of 2 gigabits or more per square inches. Furthermore, the effects of reducing grain size distribution are rare and good electromagnetic conversion characteristics cannot be obtained.
Although a magnetoresistive head is suitable for high density recording because it has a very high read sensitivity, the sensitivity relative to noises also becomes high. Therefore, in-plane magnetic recording media with low noises are required more than ever.
In order to reduce media noise and obtain good electromagnetic conversion characteristics even at high recording density, it is necessary to make crystal grain size fine and reduce grain size distribution, without degrading the hcp (11.0) orientation of the Co alloy magnetic layer.
Furthermore, even if samples of a magnetic disk drive is manufactured for use with a combination of a low noise in-plane magnetic medium and a high sensitivity magnetoresistive head, sufficient electromagnetic conversion characteristics cannot be always obtained. This may be ascribed to independent developments of magnetic heads and in-plane magnetic recording media, and to insufficient consideration about the way of head-disk combination for high recording density of the magnetic disk drive.